The most challenging ongoing water pollution problem for most cities is common stormwater runoff. But what is it and why is it such a problem?


Land in urban environments is fundamentally different from land that has been untouched. If two inches of rain falls on a forest or a field, much of it is held on leaves to evaporate, and the rest is quickly absorbed into the ground. However, in cities, much of the land is covered by impervious surfaces such as rooftops, roadways and parking lots. So when the same two inches of rain falls on a city can have a disastrous impact. The key difference is that in a city, the prevalence of impervious surfaces means that rainwater cannot be absorbed by the ground, so it flows quickly off of the land where it falls, creating a risk of flooding. Thus, cities have created sewers systems to channel rain away from the streets as quickly as possible. Usually, underground pipe systems convey the rain – what civil engineers call stormwater – directly into the nearest stream, creek, or river.

Imagine snow - clean, brilliantly white - after it has been sitting along the curb for a few days.  Our fresh, pure rainwater is similarly dirtied by the city, merging with the oil, gas, rust, harmful bacterial, toxic chemicals, trash, and dozens other types of pollution that collects on city streets, alleys, and parking lots, forming a thin film that is always there in dry weather. When stormwater rushes to the nearest stream, it takes all of those harmful elements along for the ride, dumping everything in the nearest stream. This, as you can imagine, takes a drastic toll on streams and rivers and the creatures that live in them.


Stormwater runoff can also cause massive erosion. Moving water can be magnificently powerful. City blocks vary in size, but one typical city block of 100,000 square feet covered with hard surface produces 125,000 gallons of rain in a two inch rainstorm. Twenty such blocks produce 2,500,000 gallons. That volume of water rushing from the end of a pipe into a small urban stream simply “blows out” the stream banks.

If we could stop all urban pollution and create systems that completely imitate the natural flow of water to large water bodies like the Potomac River or the Chesapeake Bay, we wouldn’t ever have to worry about the effect of stormwater runoff. This, of course, is unrealistic.

Stormwater runoff destroys the biological health of our urban streams and rivers, making them contaminated, muddy and shallow. Today, the environmental neglect that caused urban rivers to literally catch on fire in the 1960s doesn’t happen anymore. But urban rivers around the world are still muddy and polluted and some are even barren from the effect of stormwater runoff.